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1.
Dispersion states of aqueous composite Al2 O3 /ZrO2 colloidal suspensions were studied by measuring particle size distribution as a function of pH. Mutual dispersion was achieved at pH values of 2.0 to 3.5. Consolidated composites formed by colloidal filtration reflected the uniformity of the colloidal state. The mean flexural strength (896 MPa) of the sintered compacts was 1.6 times that of bodies consolidated by isostatic pressing . 相似文献
2.
Xun-Li Wang Camden R. Hubbard Kathleen B. Alexander Paul F. Becher Jaime A. Fernandez-Baca Steve Spooner 《Journal of the American Ceramic Society》1994,77(6):1569-1575
High-resolution neutron powder diffraction was used to study the residual stresses in Al2 O3 -ZrO2 (12 mol% CeO2 ) ceramic composites containing 10, 20, and 40 vol% ZrO2 (CeO2 ). The diffraction data were analyzed using the Rietveld structure refinement technique. The analysis shows that for all samples, the CeO2 -stabilized tetragonal ZrO2 particles are in tension and the Al2 O3 matrix is in compression. For both the ZrO2 particles and the Al2 O3 matrix, the average lattice strains are anisotropic and increase approximately linearly with a decrease in the corresponding phase content. It is shown that these features can be qualitatively understood by taking into consideration the thermal expansion mismatch between the ZrO2 and Al2 O3 grains. Also, for all composite samples, the diffraction peaks are broader than the instrumental resolution, indicating that the strains in these samples are inhomogeneous. From an analysis of the refined peak shape parameters, the average root-meansquare strain, which describes the distribution of the inhomogeneous strain field, was determined. Finally, the average residual stresses were evaluated from the experimentally determined average lattice strains and compared with recent results of X-ray measurements on similar composites. 相似文献
3.
Applying an Eshelby approach, the internal micromechanical stresses within an SiC-inclusion-reinforced (platelet to whisker geometries) polycrystalline alumina matrix composite were calculated. The results are compared to the experimental residual stress measurements of a SiC-whisker-reinforced Al2 O3 by Predecki, Abuhasan, and Barrett and found to be in excellent agreement. The calculations are then extended to SiC-reinforced composites with polycrystalline mullite, silicon nitride, and cordierite matrices. It is concluded that the internal stresses are significantly influenced by the inclusion geometry as well as the thermoelastic differences between the inclusion and the matrix and also the volume fraction. 相似文献
4.
Nicole R. Harlan Rosa I. Merino José I. Peña Angel Larrea Victor M. Orera Carlos González Pedro Poza Javier LLorca 《Journal of the American Ceramic Society》2002,85(8):2025-2032
Al2 O3 -ZrO2 eutectics containing 0 to 12.2 mol% Y2 O3 (with respect to zirconia) were produced by directional solidification using the laser floating zone (LFZ) method. Processing variables were chosen to obtain homogeneous, colony-free, interpenetrating microstructure for all of the compositional range, optimum from the viewpoint of mechanical properties. The amount of cubic, tetragonal, or monoclinic zirconia phases was determined using a combination of Raman and X-ray diffraction techniques. Monoclinic zirconia was present up to concentrations of 3 mol% Y2 O3 , while the amount of tetragonal zirconia gradually increased with yttria content up to 3 mol%. Cubic zirconia was the only phase detected when the yttria content reached 12 mol%. The residual stresses in alumina were measured using the shift of the ruby R lines. Compressive stresses were isotropic when measured in the samples containing tetragonal and cubic zirconia, while higher tensile, anisotropic stresses were found when monoclinic zirconia was present. They were partially relieved in the eutectic sample without yttria. These results were compared with a thermoelastic analysis based on the self-consistent model. 相似文献
5.
A method of computing the residual stress profile in quenched A12 O3 rods was developed. For these calculations, certain material parameters must be determined. Thus, strain rate was measured as a function of stress for 96% A12 O3 at 1300° to 1500°C, and the heat-transfer rates of cylindrical samples quenched in several media were determined. Using calculated temperature distributions and measured strain rates, plastic strains were computed for the entire quenching period, and these strains were converted to a profile of residual room-temperature stresses. The substantial increases in flexural strength observed in Al2 O3 after it is quenched (thermally conditioned) are considered to originate in the residual compressive surface layers. 相似文献
6.
DAVID J. GREEN 《Journal of the American Ceramic Society》1982,65(12):610-614
The internal strains asSociated with the martensitic phase transformation of zirconia were used to introduce microcracks into Al2 O3 /ZrO2 composites. The degree of transformation was found to be dependent on the volume fraction of ZrO2 and its size, the latter of which could be controlled by suitable heat treatments. The microstructural changes that occurred during the heat treatments were studied using quantitative microscopy and X-ray diffraction. For materials containing more than 7.5 vol% Zr02 , the ZrO2 particles were found to pin the Al2 O3 grain boundaries, thus limiting the Al2 O3 grain growth. The limiting grain size was found to be dependent on size and volume fraction of ZrO2 . Heat treatments for the higher volume fraction materials (>7.5 vol% ZrO2 ) caused micro-structural changes which resulted in increased amounts of monoclinic ZrO2 at room temperature; elastic modulus measurements indicated that this was occurring concurrently with microcracking. By combining the ZrO2 grain-size distributions with the X-ray analysis it was possible to calculate the critical ZrO2 size required for the transformation. The critical size was found to decrease with increasing amounts of ZrO2 . Hardness and indentation fracture toughness were measured on the composites. Grain fragmentation was observed at the edge of the indentations and microcracks were observed directly, using an AgNO3 decoration technique, near the indentations. 相似文献
7.
Values of KIc for hot-pressed AL2 O3 -ZrO2 composites were measured using notched-beam and indentation strength techniques. The results are compared with K factors at the mirror/mist boundary and at crack branching. It was found that the indentation strength technique provides a more consistent estimation of KIc , than the notched-beam technique. 相似文献
8.
The fracture strengths of sintered Al2 O3 containing 20 and 40 vol% ZrO2 (12 mol% CeO2 )—zirconia-toughened alumina (ZTA)—composites along with the fracture resistance can be increased (e.g., to ∼900 MPa and >12 Mpa·m1/2 , respectively), by increasing the mean grain size of the t -ZrO2 (and the Al2 O3 ) from ∼0.5 μm to ∼3 μm. At these lower t -ZrO2 contents, the fracture strength-fracture resistance curves show a continuous rise as opposed to the strength maxima observed in polycrystalline t -ZrO2 (12 mol% CeO2 ), CeTZP, and ZrO2 (12 mol% CeO2 ) ceramics containing ≤20 vol% Al2 O3 . The toughened composites also exhibit excellent damage resistance with fracture strengths of 500 MPa retained with surfaces containing ∼150- N Vickers indentations which produce cracks of ∼160-μm radius. Greater damage resistance correlates with an increase in the apparent R -curve response of these composites. 相似文献
9.
Bahadir Tunaboylu Joanna McKittrick W. J. Nellis Steven R. Nutt 《Journal of the American Ceramic Society》1994,77(6):1605-1612
Shock compaction of Al2 O3 -ZrO3 binary and ternary powder compositions resulted in dense, one-piece samples without visible cracks for pressures ≤12.6 GPa. Dynamic pressures were achieved by using a 6.5-m-long two-stage gas gun. It is believed that plastic deformation by dislocation slip of α-Al2 O3 partially accommodates the tensile stresses created during the release of shock pressures. A fine and narrow particle size distribution is necessary to achieve high bulk densities, but the bulk structural integrity was not strongly related to the distribution. A high-pressure phase of ZrO2 , which was formed from the monoclinic polymorph, was found at and above shock pressure of 6.3 GPa. No evidence of the orthorhombic cotunnite structure was observed. Compaction of glassy and submicrocrystalline rapidly solidified starting materials showed good structural integrity, although the bulk density was relatively low. It is not clear what the densificationhonding mechanism is in these materials, although it appears not to be plastic deformation. Microstructural analysis showed that fine and uniform microstructures are retained after compaction at appropriate dynamic pressures for all compositions, with some interparticle cohesion present. 相似文献
10.
Manuel Belmonte Jose S. Moya Pilar Miranzo 《Journal of the American Ceramic Society》1995,78(6):1661-1667
The sintering behavior of an Al2 O3 compact containing uniformly dispersed Al2 O3 platelets has been investigated. The results reveal a significant decrease in the sintering rate as well as the formation of voids and cracklike defects in the presence of nonsinterable platelets. The addition of a small amount (2 vol%) of tetragonal-ZrO2 particles enhances the sintering rate, increases end-point density (∼99.5% of theoretical density) and prevents formation of sintering defects. 相似文献
11.
Fred F. Lange Michael R. James David J. Green 《Journal of the American Ceramic Society》1983,66(2):16-C-
Residual surface stresses introduced into polycrystalline Al2 O3 during diamond grinding (320 grit) were examined by an X-ray direction technique commonly used for metals. Compressive stresses, estimated to be in the range 135 to 170 MPa and to extend 15 μm deep, were observed at the surface. It is believed that the compressive surface layer coincides with the plastic layer produced by the elastie/plastic interaction of the abrasive grains with the ceramic. The results are discussed with regard to the effect of the compressive layer on the extension of surface cracks. 相似文献
12.
The dynamic stress intensity factors, which were determined with newly developed bar impact facilities and a new data reduction procedure, for an Al2 O3 ceramic and 29 vol% SiCw /Al2 O3 composite were virtually identical, thus indicating that the short SiC whiskers were ineffective under dynamic fracture. SEM studies revealed five distinct fracture morphologies with increased percentage area of transgranular fracture in both materials with rapid crack propagation. Also, the high dynamic stress intensity factor caused multiple microscopic crack planes to form and then join as the crack advanced. 相似文献
13.
Carmen Baudín Jonas Gurauskis Antonio Javier Sánchez-Herencia Victor M. Orera 《Journal of the American Ceramic Society》2009,92(1):152-160
Fracture features, residual stresses, and zirconia transformation are studied in indentation strength specimens of alumina-Y2 O3 -stabilized zirconia (3% mol of Y2 O3 , 3YTZP) ceramics in order to analyze the extension of the indentation damage in the bulk of the specimens. Two compositions, 5 vol% 3YTZP (A5) and 40 vol% 3YTZP (A40), have been prepared by stacking tape-casted tapes and sintering. After indentation with loads ranging from 50 to 300 N, samples were fractured in four-point bending and the fracture surfaces were characterized by scanning electron microscopy. Raman and piezospectroscopic techniques were used to determine the monoclinic zirconia fraction and the residual stresses through the fracture surfaces. In the A5 composition, the indentation damage morphology was clearly half-penny, whereas the A40 composition presented Palmqvist crack formation. Zirconia transformation was only observed in the plastically deformed zones underneath the imprints whereas there were significant residual compressive stresses outside the plastic zones due to the indentation damage. The intensity of this residual compressive field was dependent on the level of zirconia transformation due to indentation damage because zirconia transformation induced tensile stress fields superimposed on the compressive stresses. 相似文献
14.
Ryan K. Roeder Kevin P. Trumble Keith J. Bowman' 《Journal of the American Ceramic Society》1997,80(1):27-36
Composites containing Ce-ZrO2 , Al2 O3 , and aligned Al2 O3 platelets were produced by centrifugal consolidation and pressureless sintering, followed by heat treatments at 1600°C for varied duration. Constituents in the consolidated microstructures were either uniformly distributed throughout or segregated into gradient layers, depending critically on platelet content. Quantitative image analysis was used to examine microstructure development with heat treatment. Changes in the volume fraction, dimensional anisotropy, and gradient of pores and platelets, as well as changes in the phase gradient, were quantified. Microstructure development was strongly dependent on the initial microstructure design attained from suspension processing. 相似文献
15.
Andrey V. Zagrebelny Erica T. Lilleodden William W. Gerberich C. Barry Carter 《Journal of the American Ceramic Society》1999,82(7):1803-1808
The deformation of thin layers of glass on crystalline materials has been examined using newly developed experimental methods for nanomechanical testing. Continuous films of anorthite (CaAl2 Si2 O8 ) were deposited onto Al2 O3 surfaces by pulsed-laser deposition. Mechanical properties such as Young's modulus and hardness were probed with a high-resolution depth-sensing indentation instrument. Nanomechanical testing, combined with AFM in situ imaging of the deformed regions, allowed force-displacement measurements and imaging of the same regions of the specimen before and immediately after indentation. This new technique eliminates any uncertainty in locating the indentation after unloading. Emphasis has been placed on examining how the Al2 O3 substrate crystallographic orientation will affect mechanical composite response of silicate-glass film/Al2 O3 system. 相似文献
16.
Koji Tsukuma Kuniyoshi Ueda Kenichi Matsushita Masahiko Shimada 《Journal of the American Ceramic Society》1985,68(2):56-C-
The temperature dependence of bending strength, fracture toughness, and Young's modulus of composite materials fabricated in the ZrO2 (Y2 O3 )-Al2 O3 system were examined. The addition of A1203 enhanced the high-temperature strength. Isostatically hot-pressed, 60 wt% ZrO2 (2 mol% Y2 O3 )/40 wt% Al2 O3 exhibited an extremely high strength, 1000 MPa, at 1000°C. 相似文献
17.
18.
Oriented samples of Al2 O3 -ZrO2 (Y2 O3 ) eutectics consisting of an alumina matrix with zirconia dispersoids were grown by directional solidification. Preferred growth directions and epitaxial relations were determined from X-ray and electron diffraction analyses. Imaging of interfaces was performed by high-resolution transmission electron microscopy on oriented platelets. Semicoherent interfaces were observed with faceting along crystallographic planes of both phases. 相似文献
19.
The effect of MgO and ZrO2 dopants, added separeately or simultaneously, on the grain size, denisity, and toughness of Al2 O3 was studied. Small ZrO2 addotoopms (<100 ppm) had little effect, whereas larger amounts decreased the sintered density. Additions of Mgo Up to the solubility limit (∼ 300 ppm) increased both density and grain size; further additions had little effect on the density but strongly reduced grain size. 相似文献
20.
Koji Tsukuma Kuniyoshi Ueda Masahiko Shimada 《Journal of the American Ceramic Society》1985,68(1):4-C-5
Composites of Al2 O3 and Y2 O3 partially-stabilized ZrO2 were isostatically hot-pressed using submicrometer powders as the starting material. The addition of Al2 O3 resulted in a large increase in bending strength. The average bending strength for a composite containing 20 wt% Al2 O3 was 2400 MPa, and its fracture toughness was 17 MN·w−3/2 相似文献